Electronic entertainment devices become more and more important. Particularly, an increasing number of users buy hard disk based audio/video players and other entertainment equipment.
Since the reduction of storage space is an important issue in the field of audio/video players, audio and video data are often stored in a compressed manner, and for security reasons in an encrypted manner.
MPEG2 is a standard for the generic coding of moving pictures and associated audio and creates a video stream out of frame data that can be arranged in a specified order called the GOP (“Group Of Pictures”) structure. An MPEG2 video bit stream is made up of a series of data frames encoding pictures. The three ways of encoding a picture are intra-coded (I picture), forward predictive (P picture) and bi-directional predictive (B picture). An intra-coded frame (I-frame) is an independently decodable frame. A forward predictive frame (P-frame) needs information of a preceding I-frame or P-frame. A bi-directional predictive frame (B-frame) is dependent on information of a preceding and/or subsequent I-frame or P-frame.
It is an interesting function in a media playback device to switch from a normal reproduction mode, in which media content is played back in a normal speed, to a trick-play reproduction mode, in which media content is played back in a modified manner, for instance with a reduced speed (“slow forward”), a still picture, or vice versa.
US 2005/0157714 A1 discloses a method for processing a packet-based scrambled stream, the method including receiving a plurality of scrambled packets in a packet stream, descrambling any of the scrambled packets, and transmitting a modified packet stream including at least one of the descrambled packets and at least one of the scrambled packets.